Last August, Alex Mandl was hosting dinner guests in his Great Falls, Va., home when Michael Dell's number popped up on his cellphone. Mr. Mandl, Dell Inc.'s lead director, excused himself for what he expected would be a routine check-in with the Dell chief executive ahead of a quarterly earnings report.

The phone call vaulted Mr. Mandl, a 69-year-old telecommunications veteran, into a high-stakes negotiation over the future of the computer maker, which brings in about $57 billion in annual revenue. It was a major test for an Austrian-born businessman best known for striking deals in the early days of the cellular industry.

Several large Dell shareholders oppose the $24.4 billion deal the board struck with Mr. Dell. One investor, billionaire Carl Icahn, has reserved the option to oust Dell's board, a person familiar with his thinking said.

Because Mr. Dell eventually allied himself with private-equity firm Silver Lake Partners to take Dell private, it has been up to Mr. Mandl to defuse worries that Mr. Dell had an inside track to buy the company on the cheap.

As head of a special board committee weighing the company's possible sale, Mr. Mandl has been an unflappable negotiator, has worked long hours, and dug into proposal details, said people close to the talks. "At times it certainly has been a full-time job and I have had to make adjustments," said Mr. Mandl in a recent interview in New York, wearing a crisp shirt and navy blazer. "There are still a number of uncertainties out there to work through, and we will do what we need to do to bring the result to the right conclusion for the shareholders," Mr. Mandl said.

While Mr. Mandl initially was skeptical of a buyout, he shuttled between Mr. Dell and Egon Durban, Silver Lake's point man in the buyout talks, to make sure Dell shareholders got the best deal possible, said people close to the talks.

The lead director also checked on Dell executives' conversations with would-be buyers to ensure managers were forthcoming about financial information. And he coaxed balking financiers back to the table when talks ran aground, they said.

He "is a very strong director and a very strong leader,'' said William Gray III, a Dell director and a former U.S. congressman. Mr. Mandl has had an unenviable task. Potential buyers doubted they could make money on a Dell buyout, directors lost faith in management financial projections, and PC sales fell faster than the board expected, said the people involved in the negotiations. Starting in March, at the end of a 45-day "go shop" period seeking alternative offers for Dell, Mr. Mandl haggled with private-equity firm Blackstone GroupBX-0.10% LP and investor Mr. Icahn over tentative bids.

Blackstone Group has dropped out of the bidding and Mr. Icahn isn't likely to buy Dell. That means Mr. Mandl now must keep Mr. Dell's deal on track and convince shareholders the $13.65-a-share offer is fair. Mr. Mandl already has spoken to shareholders wary of the buyout and plans to continue his outreach ahead of the shareholder vote. Dell shares closed Monday at $13.29.

"It has been as methodical and thorough a process as you could want, and that is due to Alex," said Ken Duberstein, another member of Dell's special committee.

Mr. Mandl also has run into critics. Some have said the director narrowed the list of potential buyers too soon, didn't dig deeply enough into alternatives to taking Dell private, and failed to lock up support from another investor, Southeastern Asset Management Inc., that has vocally opposed the deal.

"Southeastern has been working hard to secure a better outcome for our clients through a superior offer or a rejection of the current bid," the investment firm wrote in an April letter to its clients. Southeastern said in an April letter to its clients that Dell's board deliberations "focused on giving Michael Dell what he wants as opposed to pursuing credible better alternatives for shareholders and the business."

Mr. Mandl, who was born and lived in Austria until he was 15, moved to the U.S. with his parents and attended college here. He moved up the corporate ranks and joined AT&T Inc.T-0.17% in 1991 as chief financial officer.

Former colleagues credit Mr. Mandl with leading AT&T's strategy to buy its way into the cellphone market. Mr. Mandl's courtship of McCaw Cellular, for instance, resulted in a 1994 acquisition for about $11.5 billion.

The purchase "was one of the better things that AT&T did," said R. Gerard Salemme, a former McCaw Cellular and AT&T executive. "Alex was a good operator."

In 1996, Mr. Mandl jumped ship to run startup telecom firm Teligent Inc., which went bankrupt in 2001, just weeks after he was pushed out as CEO. Mr. Mandl then ran struggling French digital-security company Gemplus International SA, which he turned around before striking a $1.1 billion deal to merge with Axalto HoldingGTOMY-0.06% NV in 2005. He is its nonexecutive chairman.

A recruiter approached Mr. Mandl about joining Dell's board. He did so in 1997 and 13 years later was named independent presiding director—essentially the boss of the board on areas where Mr. Dell had a conflict. When the Round Rock, Texas, company enjoyed good times, directors rarely challenged Mr. Dell on business strategy, said people familiar with board discussions.

In more recent years, however, these people said Mr. Mandl and other directors have pressed Mr. Dell and other executives about company strategy.

A spokesman for Dell declined to comment on the board's dealings with Mr. Dell, and said Mr. Dell also declined to comment. Larry Tu, Dell's general counsel, said Mr. Mandl "has helped guide Dell through a period of unprecedented growth and change."

When Mr. Dell informed directors about the potential of taking Dell private last summer, people familiar with the matter said Mr. Mandl was among the board members skeptical about the idea amid the company's tough attempted transition from PC and server maker to a broader corporate software and tech services provider.

Through the fall, Mr. Mandl and other directors worried whether Dell's shifting internal financial outlooks meant management couldn't accurately predict future financial performance, according to a March regulatory filing. Those concerns—and a sinking PC market—helped spur directors to accept the buyout deal, said people familiar with the board deliberations.

Mr. Mandl has since spent thousands of hours working on the negotiations. In mid-January, he salvaged negotiations with Silver Lake after private-equity firms KKRKKR-0.58% & Co. and TPG dropped out—and Silver Lake was close to walking away, said people briefed on the negotiations.

Mr. Mandl called Mr. Dell and asked if the CEO planned to stay as CEO if the buyout floundered, according to a March regulatory filing. Mr. Dell said he remained committed to the company.

That same month, Mr. Mandl spent one weekend pushing for a $1.5 billion price increase the board wanted from Silver Lake. In a flurry of phone calls, Silver Lake's point man on the deal said he couldn't budge on price and that he had to tell his partners the deal likely was dead.

"Go ahead," Mr. Mandl replied, a person close to the talks said. The next day Silver Lake agreed to raise its offer for Dell. Silver Lake declined to comment. After further back and forth, Dell said publicly in February it would go private.

Now, Mr. Mandl is planning for the shareholder vote and overseeing the board's dealings with the Securities and Exchange Commission, which must clear Dell's regulatory disclosures related to the deal. He also is prepping for the possibility of curveballs to come from Mr. Icahn, Silver Lake or Dell stockholders.

Other Dell board members said Mr. Mandl has handled the situation well given the circumstances.

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